Wavelength conversion and converters are well known in the art of communications and specifically as it relates to wavelength division multiplexed lightwave networks. In general, data carried on an incident light wave at a first wavelength .lambda..sub.1 may be transferred to a second light wave at .lambda..sub.2, by modulating a continuous lightwave ("CW") at .lambda..sub.2.
Referring to FIG. 1 an example of a prior art wavelength converter is shown. A light wave at .lambda..sub.1 is shown incident at a photodetector 10, shown for purposes of illustration as a photodiode. The resulting electrical current output from detector 10 may pass through a pre-amplifier 20 such as shown, typically used to amplify the electrical signal to an intermediate level without degrading the signal to noise ratio of the signal. Thereafter, the signal is further amplified as it passes through amplifier 30 and followed by the last amplifier, also referred to as driver 40. The output from driver 40 and CW light at .lambda..sub.2 are input to modulator 50. Modulator 50 takes the electrical data from driver 40, modulates the CW light at .lambda..sub.2 and outputs a lightwave at .lambda..sub.2 carrying the original incident data.
The amplification stage between detection and modulation is necessary because the incident signal may be at a low voltage level on the order of millivolts, while the modulator will typically require anywhere from 1-6 volts peak to peak. More specifically, a modulator such as 50 shown in FIG. 1 may be manufactured from Lithium Niobate, in which case it will require 3 to 6 volts peak to peak. Alternatively, the modulator may be manufactured from semiconductor material in which case 1-3 volts should be sufficient. In either case, amplification is necessary.
The drawback of the wavelength converter of FIG. 1 is that it includes electrical connections and devices. The electrical components are sensitive to data transmission rates and should have sufficient electrical bandwidth to operate at the data transmission rate. As bit rates increase, obtaining amplifiers with the proper bandwidth becomes a challenge. While connections can be fabricated to accept the higher bit rates, such as in the range of gigabytes, they must also be compatible with the amplifiers they inter-connect which traditionally have ratings of 50 ohms. Accordingly, in light of ever increasing demand for higher data transfer rates, it is desirable to reduce the constraints created by the electrical elements in the circuit, e.g. transistors, capacitors and wiring.
Referring to FIG. 2 an alternative wavelength converter is shown which is known in the art as a Semiconductor Optical Amplifier ("SOA"). SOA 60 is a single semiconductor material whose properties allow for the amplification and modulation of an incident light wave into a second wavelength. SOA 60 accepts two inputs: the data at .lambda..sub.1 and the CW at .lambda..sub.2. Direct current voltage is applied to the device and the data is output at an amplified level and at .lambda..sub.2. The process of SOA 60 is also referred to as cross gain modulation and is more fully described in S. J. B. Yoo, "Wavelength Conversion Technologies for WDM Network Applications," in 14 Journal of Lightwave Technology p. 955 (1996), hereby incorporated by reference as if fully set forth herein.
While for certain applications an SOA may be the device of choice, as compared with the opto/electronic wavelength converter of FIG. 1, it suffers certain drawbacks, including the introduction of certain non linear noise into the signal.